Method and apparatus for generating an uplink reference signal sequence in a wireless communication system

ABSTRACT

A method of receiving, by a base station, a reference signal in a wireless communication system. The method includes transmitting a cell-specific sequence group hopping parameter to a plurality of user equipments (UEs) in a cell. The cell-specific sequence group hopping parameter is used to enable a sequence group hopping for the plurality of UEs in the cell. The method further includes transmitting a UE-specific sequence group hopping parameter to a certain UE, among the plurality of UEs. The UE-specific sequence group hopping parameter is used to disable the sequence group hopping, enabled by the cell-specific SGH parameter, for the certain UE. The method further includes receiving a reference signal, which is generated based on a sequence group number, from the certain UE. The sequence group number is determined by the UE-specific sequence group hopping parameter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent application Ser. No. 13/521,017 filed on Aug. 1, 2012, which is the National Phase of PCT/KR2011/000110 filed on January 7, 2011, which claims priority to U.S. Provisional Application Nos. 61/292,868, 61/328,189, 61/334,555, and 61/345,154 filed on Jan. 7, 2010, Apr. 27, 2010, May 13, 2010, and May 17, 2010, and which claims priority to Patent Application No. 10-2011-0001669 filed in the Republic of Korea on Jan. 7, 2011, the entire contents of all of the above applications are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless communication, and more particularly, to a method and apparatus for generating a reference signal sequence in a wireless communication system.

2. Discussion of the Related Art

Multiple-input multiple-output (MIMO) technology can be used to improve the efficiency of data transmission and reception using multiple transmission antennas and multiple reception antennas. MIMO technology may include a space frequency block code (SFBC), a space time block code (STBC), a cyclic delay diversity (CDD), a frequency switched transmit diversity (FSTD), a time switched transmit diversity (TSTD), a precoding vector switching (PVS), spatial multiplexing (SM) for implementing diversity. An MIMO channel matrix according to the number of reception antennas and the number of transmission antennas can be decomposed into a number of independent channels. Each of the independent channels is called a layer or stream. The number of layers is called a rank.

In wireless communication systems, it is necessary to estimate an uplink channel or a downlink channel for the purpose of the transmission and reception of data, the acquisition of system synchronization, and the feedback of channel information. In wireless communication system environments, fading is generated because of multi-path time latency. A process of restoring a transmit signal by compensating for the distortion of the signal resulting from a sudden change in the environment due to such fading is referred to as channel estimation. It is also necessary to measure the state of a channel for a cell to which a user equipment belongs or other cells. To estimate a channel or measure the state of a channel, a reference signal (RS) which is known to both a transmitter and a receiver can be used.

A subcarrier used to transmit the reference signal is referred to as a reference signal subcarrier, and a subcarrier used to transmit data is referred to as a data subcarrier. In an OFDM system, a method of assigning the reference signal includes a method of assigning the reference signal to all the subcarriers and a method of assigning the reference signal between data subcarriers. The method of assigning the reference signal to all the subcarriers is performed using a signal including only the reference signal, such as a preamble signal, in order to obtain the throughput of channel estimation. If this method is used, the performance of channel estimation can be improved as compared with the method of assigning the reference signal between data subcarriers because the density of reference signals is in general high. However, since the amount of transmitted data is small in the method of assigning the reference signal to all the subcarriers, the method of assigning the reference signal between data subcarriers is used in order to increase the amount of transmitted data. If the method of assigning the reference signal between data subcarriers is used, the performance of channel estimation can be deteriorated because the density of reference signals is low. Accordingly, the reference signals should be properly arranged in order to minimize such deterioration.

A receiver can estimate a channel by separating information about a reference signal from a received signal because it knows the information about a reference signal and can accurately estimate data, transmitted by a transmit stage, by compensating for an estimated channel value. Assuming that the reference signal transmitted by the transmitter is p, channel information experienced by the reference signal during transmission is h, thermal noise occurring in the receiver is n, and the signal received by the receiver is y, it can result in y=h·p+n. Here, since the receiver already knows the reference signal p, it can estimate a channel information value ĥ using Equation 1 in the case in which a Least Square (LS) method is used.

ĥ=y/p=h+n/p=h+{circumflex over (n)}  [Equation 1]

The accuracy of the channel estimation value ĥ estimated using the reference signal p is determined by the value {circumflex over (n)}. To accurately estimate the value h, the value {circumflex over (n)} must converge on 0. To this end, the influence of the value {circumflex over (n)} has to be minimized by estimating a channel using a large number of reference signals. A variety of algorithms for a better channel estimation performance may exist.

In order to minimize inter-cell interference (ICI) in transmitting a reference signal, sequence group hopping (SGH) or sequence hopping (SH) may be applied to a reference signal sequence. When the SGH is applied, the sequence group index of a reference signal sequence transmitted in each slot may he changed.

In a multi-user (MU) MIMO environment, in order to guarantee orthogonality between reference signals transmitted by a plurality of UEs, an orthogonal covering code (OCC) may be used. When the OCC is applied, the improvement of orthogonality and throughput can be guaranteed. Meanwhile, in an MU-MIMO environment, a plurality of UEs may use different bandwidths. If the OCC is applied while the SCH is performed on reference signals transmitted by the plurality of UEs having different bandwidths, the complexity of cell planning is increased. That is, it is difficult to guarantee orthogonality the reference signals transmitted by the plurality of UEs.

Accordingly, there is a need for another method to indicate whether to perform SGH or SH on a reference signal sequence.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for generating a reference signal sequence in a wireless communication system.

In an aspect, a method of generating, by a user equipment (UE), a reference signal sequence in a wireless communication system is provided. The method includes receiving a UE-specific sequence group hopping (SGH) parameter specified to the UE; and generating the reference signal sequence based on a base sequence for every slot, wherein the base sequence is classified according to a sequence-group number and a base sequence number which are determined for every slot by the UE-specific SGH parameter indicating whether to perform SGH.

The UE-specific SGH parameter may be transmitted through a higher layer.

The reference signal sequence may be a sequence of a demodulation reference signal (DMRS) that uses a physical uplink shared channel (PUSCH) resources and demodulates a signal.

When the UE-specific SGH parameter indicates that SGH is not performed, a sequence-group number of slots within one subframe and a base sequence number within a sequence group may be identical with each other.

When the UE-specific SGH parameter indicates that sequence hopping (SH) is not performed, a sequence-group number of slots within one subframe and a base sequence number within a sequence group may be identical with each other.

When the UE-specific SGH parameter indicates that SGH is not performed, sequence-group numbers of all slots within a frame may be identical with each other.

The method may further include receiving a cell-specific SGH parameter indicating whether to perform SGH or a cell-specific SH parameter indicating whether to perform SH. When the cell-specific GH parameter indicates that SGH is performed, the UE-specific SGH parameter may override the cell-specific SGH parameter in indicating whether to perform SGH. When the cell-specific SH parameter indicates that SH is performed, the UE-specific SGH parameter may override the cell-specific SH parameter in indicating whether to perform SH.

The method may further include transmitting the reference signal sequence by mapping the reference signal sequence to a subcarrier.

The reference signal sequence may be generated further based on a cyclic shift.

The base sequence may be based on a Zadoff-Chu (ZC) sequence.

An orthogonal covering code (OCC) may be applied to the reference signal sequence. Whether to apply the OCC may be indicated by an OCC index transmitted through a higher layer.

In another aspect, an apparatus for generating a reference signal sequence is provided. The apparatus includes a radio frequency (RF) unit configured to receive a user equipment (UE)-specific sequence group hopping (SGH) parameter, and a processor coupled to the RF unit and configured to generate the signal sequence based on a base sequence for every slot, wherein the base sequence is classified according to a sequence-group number and a base sequence number which are determined for every slot by the UE-specific SGH parameter indicating whether to perform SGH.

In a MU-MIMO environment, orthogonality between a plurality of UEs using different bandwidths can be guaranteed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system.

FIG. 2 shows the structure of a radio frame in 3GPP LTE.

FIG. 3 shows an example of a resource grid of a single downlink slot.

FIG. 4 shows the structure of a downlink subframe.

FIG. 5 shows the structure of an uplink subframe.

FIG. 6 shows an example of the structure of a transmitter in an SC-FDMA system.

FIG. 7 shows an example of a scheme in which the subcarrier mapper maps the complex-valued symbols to the respective subcarriers of the frequency domain.

FIG. 8 shows an example of the structure of a reference signal transmitter for demodulation.

FIG. 9 shows examples of a subframe through which a reference signal is transmitted.

FIG. 10 shows an example of a transmitter using the clustered DFT-s OFDM transmission scheme.

FIG. 11 shows another example of a transmitter using the clustered DFT-s OFDM transmission scheme.

FIG. 12 is another example of a transmitter using the clustered DFT-s OFDM transmission scheme.

FIG. 13 shows an example where the OCC is applied to a reference signal.

FIG. 14 is an example where a plurality of UEs performs MU-MIMO transmission using different bandwidths.

FIG. 15 is an example where SGH and SH are not performed by the proposed UE-specific SGH parameter.

FIG. 16 is an embodiment of a proposed method of generating a reference signal sequence.

FIG. 17 is a block diagram showing a BS and UE in which the embodiments of the present invention are implemented.

DETAILED DESCRIPTION OF THE INVENTION

The following technique may be used for various wireless communication systems such as code division multiple access (CDMA), a frequency division multiple access (FDMA), time division multiple access (TDMA), orthogonal frequency division multiple access (OFDMA), single carrier-frequency division multiple access (SC-FDMA), and the like. The CDMA may be implemented as a radio technology such as universal terrestrial radio access (UTRA) or CDMA2000. The TDMA may be implemented as a radio technology such as a global system for mobile communications (GSM)/general packet radio service (GPRS)/enhanced data rates for GSM evolution (EDGE). The OFDMA may be implemented by a radio technology such as institute of electrical and electronics engineers (IEEE) 802.11 (Wi-Ei) IEEE 802.16 (WiMAX), IEEE 802.20, E-UTRA (evolved UTRA), and the like. IEEE 802.16m, an evolution of IEEE 802.16e, provides backward compatibility with a system based on IEEE 802.16e. The UTRA is part of a universal mobile telecommunications system (UMTS). 3GPP (3rd generation partnership project) LTE (long term evolution) is part of an evolved UMTS (E-UMTS) using the E-UTRA, which employs the OFDMA in downlink and the SC-EDMA in uplink. LTE-A (advanced) is an evolution of 3GPP LTE.

Hereinafter, for clarification, LTE-A will be largely described, but the technical concept of the present invention is not meant to be limited thereto.

FIG. 1 shows a wireless communication system.

The wireless communication system 10 includes at least one base station (BS) 11. Respective BSs 11 provide a communication service to particular geographical areas 15 a, 15 b, and 15 c (which are generally called cells). Each cell may be divided into a plurality of areas (which are called sectors). A user equipment (UE) 12 may be fixed or mobile and may be referred to by other names such as MS (mobile station), MT (mobile terminal), UT (user terminal), SS (subscriber station), wireless device, PDA (personal digital assistant), wireless modem, handheld device. The BS 11 generally refers to a fixed station that communicates with the UE 12 and may be called by other names such as eNB (evolved-NodeB), BTS (base transceiver system), access point (AP), etc.

In general, a UE belongs to one cell, and the cell to which a UE belongs is called a serving cell. A BS providing a communication service to the serving cell is called a serving BS. The wireless communication system is a cellular system, so a different cell adjacent to the serving cell exists. The different cell adjacent to the serving cell is called a neighbor cell. A BS providing a communication service to the neighbor cell is called a neighbor BS. The serving cell and the neighbor cell are relatively determined based on a UE.

This technique can be used for downlink or uplink. In general, downlink refers to communication from the BS 11 to the UE 12, and uplink refers to communication from the UE 12 to the BS 11. In downlink, a transmitter may be part of the BS 11 and a receiver may be part of the UE 12. In uplink, a transmitter may be part of the UB 12 and a receiver may be part of the BS 11.

The wireless communication system may be any one of a multiple-input multiple-output (MIMO) system, a multiple-input single-output (MISO) system, a single-input single-output (SISO) system, and a single-input multiple-output (SIMO) system. The MIMO system uses a plurality of transmission antennas and a plurality of reception antennas. The MISO system uses a plurality of transmission antennas and a single reception antenna. The SISO system uses a single transmission antenna and a single reception antenna. The SIMO system uses a single transmission antenna and a plurality of reception antennas. Hereinafter, a transmission antenna refers to a physical or logical antenna used for transmitting a signal or a stream, and a reception antenna refers to a physical or logical antenna used for receiving a signal or a stream.

FIG. 2 shows the structure of a radio frame in 3GPP LTE.

It may be referred to Paragraph 5 of “Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation (Release 8)” to 3GPP (3rd generation partnership project) TS 36.211 V8.2.0 (2008-03). Referring to FIG. 2, the radio frame includes 10 subframes, and one subframe includes two slots. The slots in the radio frame are numbered by #0 to #19. A time taken for transmitting one subframe is called a transmission time interval (TTI). The TTI may be a scheduling unit for a data transmission. For example, a radio frame may have a length of 10 ms, a subframe may have a length of 1 ms, and a slot may have a length of 0.5 ms.

One slot includes a plurality of orthogonal frequency division multiplexing (OFDM) symbols in a time domain and a plurality of subcarriers in a frequency domain. Since 3GPP LTE uses OFDMA in downlink, the OFDM symbols are used to express a symbol period. The OFDM symbols may be called by other names depending on a multiple-access scheme. For example, when a single carrier frequency division multiple access (SC-FDMA) is in use as an uplink multi-access scheme, the OFDM symbols may be called SC-FDMA symbols. A resource block (RB), a resource allocation unit, includes a plurality of continuous subcarriers in a slot. The structure of the radio frame is merely an example. Namely, the number of subframes included in a radio frame, the number of slots included in a subframe, or the number of OFDM symbols included in a slot may vary.

3GPP LTE defines that one slot includes seven OFDM symbols in a normal cyclic prefix (CP) and one slot includes six OFDM symbols in an extended CP.

The wireless communication system may be divided into a frequency division duplex (FDD) scheme and a time division duplex (TDD) scheme. According to the FDD scheme, an uplink transmission and a downlink transmission are made at different frequency bands. According to the TDD scheme, an uplink transmission and a downlink transmission are made during different periods of time at the same frequency band. A channel response of the TDD scheme is substantially reciprocal. This means that a downlink channel response and an uplink channel response are almost the same in a given frequency band. Thus, the TDD-based wireless communication system is advantageous in that the downlink channel response can be obtained from the uplink channel response. In the TDD scheme, the entire frequency band is time-divided for uplink and downlink transmissions, so a downlink transmission by the BS and an uplink transmission by the UE can be simultaneously performed. In a TDD system in which an uplink transmission and a downlink transmission are discriminated in units of subframes, the uplink transmission and the downlink transmission are performed in different subframes.

FIG. 3 shows an example of a resource grid of a single downlink slot.

A downlink slot includes a plurality of OFDM symbols in the time domain and N_(RB) number of resource blocks (RBs) in the frequency domain. The N_(RB) number of resource blocks included in the downlink slot is dependent upon a downlink transmission bandwidth set in a cell. For example, in an LTE system, N_(KB) may be any one of 60 to 110. One resource block includes a plurality of subcarriers in the frequency domain. An uplink slot may have the same structure as that of the downlink slot.

Each element on the resource grid is called a resource element. The resource elements on the resource grid can be discriminated by a pair of indexes (k,l) in the slot. Here, k (k=0, . . . , N_(RB)×12−1) is a subcarrier index in the frequency domain, and l is an OFDM symbol index in the time domain.

Here, it is illustrated that one resource block includes 7×12 resource elements made up of seven OFDM symbols in the time domain and twelve subcarriers in the frequency domain, but the number of OFDM symbols and the number of subcarriers in the resource block are not limited thereto. The number of OFDM symbols and the number of subcarriers may vary depending on the length of a cyclic prefix (CP), frequency spacing, and the like. For example, in case of a normal CP, the number of OFDM symbols is 7, and in case of an extended CP, the number of OFDM symbols is 6, One of 128, 256, 512, 1024, 1536, and 2048 may be selectively used as the number of subcarriers in one OFDM symbol.

FIG. 4 shows the structure of a downlink subframe.

A downlink subframe includes two slots in the time domain, and each of the slots includes seven OFDM symbols in the normal CP. First three OFDM symbols (maximum four OFDM symbols with respect to a 1.4 MHz bandwidth) of a first slot in the subframe corresponds to a control region to which control channels are allocated, and the other remaining OFDM symbols correspond to a data region to which a physical downlink shared channel (PDSCH) is allocated.

The PDCCH may carry a transmission format and a resource allocation of a downlink shared channel (DL-SCH), resource allocation information of an uplink shared channel (UL-SCH), paging information on a PCH, system information on a DL-SCH, a resource allocation of an higher layer control message such as a random access response transmitted via a PDSCH, a set of transmission power control commands with respect to individual UEs in a certain UE group, an activation of a voice over internet protocol (VoIP), and the like. A plurality of PDCCHs may be transmitted in the control region, and a UE can monitor a plurality of PDCCHs. The PDCCHs are transmitted on one or an aggregation of a plurality of consecutive control channel elements (CCE). The CCE is a logical allocation unit used to provide a coding rate according to the state of a wireless channel. The CCE corresponds to a plurality of resource element groups. The format of the PDCCH and an available number of bits of the PDCCH are determined according to an associative relation between the number of the CCEs and a coding rate provided by the CCEs.

The BS determines a PDCCH format according to a DCI to be transmitted to the UE, and attaches a cyclic redundancy check (CRC) to the DCI. A unique radio network temporary identifier (RNTI) is masked on the CRC according to the owner or the purpose of the PDCCH. In case of a PDCCH for a particular UE, a unique identifier, e.g., a cell-RNTI (C-RNTI), of the UE, may be masked on the CRC. Or, in case of a PDCCH for a paging message, a paging indication identifier, e.g., a paging-RNTI (P-RNTI), may be masked on the CRC. In case of a PDCCH for a system information block (SIB), a system information identifier, e.g., a system information-RNTI (SI-RNTI), may be masked on the CRC. In order to indicate a random access response, i.e., a response to a transmission of a random access preamble of the UE, a random access-RNTI (RA-RNTI) may be masked on the CRC.

FIG. 5 shows the structure of an uplink subframe.

An uplink subframe may be divided into a control region and a data region in the frequency domain. A physical uplink control channel (PUCCH) for transmitting uplink control information is allocated to the control region. A physical uplink shared channel (PUCCH.) for transmitting data is allocated to the data region. If indicated by a higher layer, the user equipment may support simultaneous transmission of the PUCCH and the PUSCH.

The PUCCH for one UE is allocated in an RB pair. RBs belonging to the RB pair occupy different subcarriers in each of a 1^(st) slot and a 2^(nd) slot. A frequency occupied by the RBs belonging to the RB pair allocated to the PUCCH changes at a slot boundary. This is called that the RB pair allocated to the PUCCH is frequency-hopped at a slot boundary. Since the UE transmits UL control information over time through different subcarriers, a frequency diversity gain can be obtained. In the figure, m is a location index indicating a logical frequency-domain location of the RB pair allocated to the PUCCH in the subframe.

Uplink control information transmitted on the PUCCH may include a HARQ ACK/NACK, a channel quality indicator (CQI) indicating the state of a downlink channel, a scheduling request (SR) which is an uplink radio resource allocation request, and the like.

The PUSCH is mapped to a uplink shared channel (UL-SCH), a transport channel. Uplink data transmitted on the PUSCH may be a transport block, a data block for the UL-SCH transmitted during the TTI. The transport block may be user information. Or, the uplink data may be multiplexed data. The multiplexed data may be data obtained by multiplexing the transport block for the UL-SCH and control information. For example, control information multiplexed to data may include a CQI, a preceding matrix indicator (PMI), an HARQ, a rank indicator (RI), or the like. Or the uplink data may include only control information.

FIG. 6 shows an example of the structure of a transmitter in an SC-FDMA system.

Referring to FIG. 6, the transmitter 50 includes a discrete Fourier transform (DPT) unit 51, a subcarrier mapper 52, an inverse fast Fourier transform (IFFT) unit 53, and a cyclic prefix (CP) insertion unit 54. The transmitter 50 may include a scramble unit (not shown), a modulation mapper (not shown), a layer mapper (not shown), and a layer permutator (not shown), which may be placed in front of the DFT unit 51.

The DFT unit 51 outputs complex-valued symbols by performing DFT on input symbols. For example, when Ntx symbols are input (where Ntx is a natural number), a DFT size is Ntx. The DFT unit 51 may be called a transform precoder. The subcarrier mapper 52 maps the complex-valued symbols to the respective subcarriers of the frequency domain. The complex-valued symbols may be mapped to resource elements corresponding to a resource block allocated for data transmission. The subcarrier mapper 52 may be called a resource element mapper. The IFFT unit 53 outputs a baseband signal for data (that is, a time domain signal) by performing IFFT on the input symbols. The CP insertion unit 54 copies some of the rear part of the baseband signal for data and inserts the copied parts into the former part of the baseband signal for data. Orthogonality may be maintained even in a multi-path channel because inter-symbol interference (ISI) and inter-carrier interference (ICI) are prevented through CP insertion.

FIG. 7 shows an example of a scheme in which the subcarrier mapper maps the complex-valued symbols to the respective subcarriers of the frequency domain. Referring to FIG. 7( a), the subcarrier mapper maps the complex-valued symbols, outputted from the DFT unit, to subcarriers contiguous to each other in the frequency domain. ‘0’ is inserted into subcarriers to which the complex-valued symbols are not mapped. This is called localized mapping. In a 3GPP LIE system, a localized mapping scheme is used. Referring to FIG. 7( b), the subcarrier mapper inserts an (L−1) number of ‘0’ every two contiguous complex-valued symbols which are outputted from the DFT unit (L is a natural number). That is, the complex-valued symbols outputted from the DFT unit are mapped to subcarriers distributed at equal intervals in the frequency domain. This is called distributed mapping. If the subcarrier mapper uses the localized mapping scheme as in FIG. 7( a) or the distributed mapping scheme as in FIG. 7( b), a single carrier characteristic is maintained.

FIG. 8 shows an example of the structure of a reference signal transmitter for demodulation.

Referring to FIG. 8, the reference signal transmitter 60 includes a subcarrier mapper 61, an IFFT unit 62, and a CP insertion unit 63. Unlike the transmitter 50 of FIG. 6, in the reference signal transmitter 60, a reference signal is directly generated in the frequency domain without passing through the DFT unit 51 and then mapped to subcarriers through the subcarrier mapper 61. Here, the subcarrier mapper may map the reference signal to the subcarriers using the localized mapping scheme of FIG. 7( a).

FIG. 9 shows examples of a subframe through which a reference signal is transmitted. The structure of a subframe in FIG. 9( a) shows a case of a normal CP. The subframe includes a first slot and a second slot. Each of the first slot and the second slot includes 7 OFDM symbols. The 14 OFDM symbols within the subframe are assigned respective symbol indices 0 to 13. Reference signals may be transmitted through the OFDM symbols having the symbol indices 3 and 10. The reference signals may be transmitted using a sequence. A Zadoff-Chu (ZC) sequence may be used as the reference signal sequence. A variety of ZC sequences may be generated according to a root index and a cyclic shift value. A BS may estimate the channels of a plurality of UEs through an orthogonal sequence or a quasi-orthogonal sequence by allocating different cyclic shift values to the UEs. The positions of the reference signals occupied in the two slots within the subframe in the frequency domain may be identical with each other or different from each other. In the two slots, the same reference signal sequence is used. Data may be transmitted through the remaining SC-FDMA symbols other than the SC-FDMA symbols through which the reference signals are transmitted. The structure of a subframe in FIG. 9( b) shows a case of an extended CP. The subframe includes a first slot and a second slot. Each of the first slot and the second slot includes 6 SC-FDMA symbols. The 12 SC-FDMA symbols within the subframe are assigned symbol indices 0 to 11. Reference signals are transmitted through the SC-FDMA symbols having the symbol indices 2 and 8. Data is transmitted through the remaining SC-FDMA symbols other than the SC-FDMA symbols through which the reference signals are transmitted.

Although not shown in FIG. 9, a sounding reference signal (SRS) may be transmitted through the OFDM symbols within the subframe. The SRS is a reference signal for UL scheduling which is transmitted from UE to a BS. The BS estimates a UL channel through the received SRS and uses the estimated UL channel in UL scheduling.

A clustered DPT-s OFDM transmission scheme is a modification of the existing SC-FDMA transmission scheme and is a method of dividing data symbols, subjected to a precoder, into a plurality of subblocks, separating the subblocks, and mapping the subblocks in the frequency domain.

FIG. 10 shows an example of a transmitter using the clustered DFT-s OFDM transmission scheme. Referring to FIG. 10, the transmitter 70 includes a DFT unit 71, a subcarrier mapper 72, an IFFT unit 73, and a CP insertion unit 74. The transmitter 70 may further include a scramble unit (not shown), a modulation mapper (not shown), a layer mapper (not shown), and a layer permutator (not shown), which may be placed in front of the DFT unit 71.

Complex-valued symbols outputted from the DFT unit 71 are divided into N subblocks (N is a natural number). The N subblocks may be represented by a subblock #1, a subblock #2, . . . , a subblock #N. The subcarrier mapper 72 distributes the N subblocks in the frequency domain and maps the N subblocks to subcarriers. The NULL may be inserted every two contiguous subblocks. The complex-valued symbols within one subblock may be mapped to subcarriers contiguous to each other in the frequency domain. That is, the localized mapping scheme may be used within one subblock.

The transmitter 70 of FIG. 10 may be used both in a single carrier transmitter or a multi-carrier transmitter. If the transmitter 70 is used in the single carrier transmitter, all the N subblocks correspond to one carrier. If the transmitter 70 is used in the multi-carrier transmitter, each of the N subblocks may correspond to one carrier. Alternatively, even if the transmitter 70 is used in the multi-carrier transmitter, a plurality of subblocks of the N subblocks may correspond to one carrier. Meanwhile, in the transmitter 70 of FIG. 10, a time domain signal is generated through one IFFT unit 73. Accordingly, in order for the transmitter 70 of FIG. 10 to be used in a multi-carrier transmitter, subcarrier intervals between contiguous carriers in a contiguous carrier allocation situation must be aligned.

FIG. 11 shows another example of a transmitter using the clustered DFT-s OFDM transmission scheme. Referring to FIG. 11, the transmitter 80 includes a DFT unit 81, a subcarrier mapper 82, a plurality of IFFT units 83-1, 83-2, . . . , 83-N (N is a natural number), and a CP insertion unit 84. The transmitter 80 may further include a scramble unit (not shown), a modulation mapper (not shown), a layer mapper (not shown), and a layer permutator (not shown), which may be placed in front of the DFT unit 71.

IFFT is individually performed on each of N subblocks. An n^(th) IFFT unit 38-n outputs an n^(th) baseband signal (n=1, 2, . . . , N) by performing IFFT on a subblock #n. The n^(th) baseband signal is multiplied by an n^(th) carrier signal to produce an n^(th) radio signal. After the N radio signals generated from the N subblocks are added, a CP is inserted by the CP insertion unit 314. The transmitter 80 of FIG. 11 may be used in a discontiguous carrier allocation situation where carriers allocated to the transmitter are not contiguous to each other.

FIG. 12 is another example of a transmitter using the clustered DFT-s OFDM transmission scheme. FIG. 12 is a chunk-specific DFT-s OFDM system performing DFT precoding on a chunk basis. This may be called Nx SC-FDMA. Referring to FIG. 12, the transmitter 90 includes a code block division unit 91, a chunk division unit 92, a plurality of channel coding units 93-1, . . . , 93-N, a plurality of modulators 94-1, . . . , 4914-N, a plurality of DFT units 95-1, . . . , 95-N, a plurality of subcarrier mappers 96-1, . . . , 96-N, a plurality of WIT units 97-1, . . . , 97-N, and a CP insertion unit 98. Here, N may be the number of multiple carriers used by a multi-carrier transmitter. Each of the channel coding units 93-1, . . . , 93-N may include a scramble unit (not shown). The modulators 94-1, . . . , 94-N may also be called modulation mappers. The transmitter 90 may further include a layer mapper (not shown) and a layer permutator (not shown) which may be placed in front of the DFT units 95-1, . . . , 95-N.

The code block division unit 91 divides a transmission block into a plurality of code blocks. The chunk division unit 92 divides the code blocks into a plurality of chunks. Here, the code block may be data transmitted by a multi-carrier transmitter, and the chunk may be a data piece transmitted through one of multiple carriers. The transmitter 90 performs DFT on a chunk basis. The transmitter 90 may be used in a discontiguous carrier allocation situation or a contiguous carrier allocation situation.

A UL reference signal is described below.

In general, the reference signal is transmitted in the form of a sequence. A specific sequence may be used as the reference signal sequence without a special limit. A phase shift keying (PSK)-based computer generated sequence may he used as the reference signal sequence. Examples of PSK include binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK). Alternatively, a constant amplitude zero auto-correlation (CAZAC) sequence may be used as the reference signal sequence. Examples of the CAZAC sequence include a Zadoff-Chu (ZC)-based sequence, a ZC sequence with cyclic extension, and a ZC sequence with truncation. Alternatively, a pseudo-random (PN) sequence may be used as the reference signal sequence. Examples of the PN sequence include an m-sequence, a computer-generated sequence, a gold sequence, and a Kasami sequence. A cyclically shifted sequence may be used as the reference signal sequence.

A UL reference signal may be divided into a demodulation reference signal (DMRS) and a sounding reference signal (SRS). The DMRS is a reference signal used in channel estimation for the demodulation of a received signal. The DMRS may be associated with the transmission of a PUSCH or PUCCH. The SRS is a reference signal transmitted from a UE to a BS for UL scheduling. The BS estimates an UL channel through the received SRS and uses the estimated UL channel in UL scheduling. The SRS is not associated with the transmission of a PUSCH or PUCCH. The same kind of a basic sequence may be used for the DMRS and the SRS. Meanwhile, in UL multi-antenna transmission, preceding applied to the DMRS may be the same as preceding applied to a PUSCH. Cyclic shift separation is a primary scheme for multiplexing the DMRS. In an LTE-A system, the SRS may not be precoded and may be an antenna-specific reference signal.

A reference signal sequence r_(u,v) ^((α))(n) may be defined based on a basic sequence b_(u,v)(n) and a cyclic shift a according to Equation 2.

r _(u,v) ^((α))(n)=e ^(jαn) b _(u,v)(n), 0≦n<M _(sc) ^(RS)   [Equation 2]

In Equation 2, M_(sc) ^(RS)(1≦m≦N_(RB) ^(max,UL)) is the length of the reference signal sequence and M_(sc) ^(RS)=m*N_(sc) ^(RB). N_(sc) ^(RB) is the size of a resource block, indicated by the number of subcarriers in the frequency domain. N_(RB) ^(max,UL) indicates a maximum value of a UL bandwidth indicated by a multiple of N_(sc) ^(RB). A plurality of reference signal sequences may be defined by differently applying a cyclic shift value a from one basic sequence.

A basic sequence b_(u,v)(n) is divided into a plurality of groups. Here, u ∈ {0, 1, . . . , 29} indicates a group index, and v indicates a basic sequence index within the group. The basic sequence depends on the length M_(sc) ^(RS) of the basic sequence. Each group includes a basic sequence (v=0) having a length of M_(sc) ^(RS) for m (6≦m≦n_(RB) ^(max,UL)) and includes 2 basic sequences (v=0,1) having a length of M_(sc) ^(RS) for m (6≦m≦n_(RB) ^(max,UL)). The sequence group index u and the basic sequence index v within a group may vary according to time as in group hopping or sequence hopping.

Furthermore, if the length of the reference signal sequence is 3N_(sc) ^(RB) or higher, the basic sequence may be defined by Equation 3.

b _(u,v)(n)=x _(q)(nmod N _(ZC) ^(RS)),   [Equation 3]

In Equation 3, q indicates a root index of a Zadoff-Chu (ZC) sequence. N_(ZC) ^(RS) is the length of the ZC sequence and may be a maximum prime number smaller than M_(sc) ^(RS). The ZC sequence having the root index q may be defined by Equation 4.

$\begin{matrix} {{{x_{q}(m)} = ^{{- j}\frac{\pi \; {qm}{({m + 1})}}{N_{ZC}^{RS}}}},{0 \leq m \leq {N_{ZC}^{RS} - 1}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack \end{matrix}$

q may be given by Equation 5.

q=└ q+½┘+v·(−1)^(└2 q┘)

q=N _(ZC) ^(RS)·(u+1)/31   [Equation 5]

If the length of the reference signal sequence is 3N_(sc) ^(RB) or less, the basic sequence may be defined by Equation 6.

b _(u,v)(n)=e ^(eφ(n)π/4), 0≦n≦M _(sc) ^(RS)−1   [Equation 6]

Table 1 is an example where φ(n) is defined when MscRS=NscRB.

TABLE 1 φ(0), . . . , φ(11) 0 −1 1 3 −3 3 3 1 1 3 1 −3 3 1 1 1 3 3 3 −1 1 −3 −3 1 −3 3 2 1 1 −3 −3 −3 −1 −3 −3 1 −3 1 −1 3 −1 1 1 1 1 −1 −3 −3 1 −3 3 −1 4 −1 3 1 −1 1 −1 −3 −1 1 −1 1 3 5 1 −3 3 −1 −1 1 1 −1 −1 3 −3 1 6 −1 3 −3 −3 −3 3 1 −1 3 3 −3 1 7 −3 −1 −1 −1 1 −3 3 −1 1 −3 3 1 8 1 −3 3 1 −1 −1 −1 1 1 3 −1 1 9 1 −3 −1 3 3 −1 −3 1 1 1 1 1 10 −1 3 −1 1 1 −3 −3 −1 −3 −3 3 −1 11 3 1 −1 −1 3 3 −3 1 3 1 3 3 12 1 −3 1 1 −3 1 1 1 −3 −3 −3 1 13 3 3 −3 3 −3 1 1 3 −1 −3 3 3 14 −3 1 −1 −3 −1 3 1 3 3 3 −1 1 15 3 −1 1 −3 −1 −1 1 1 3 1 −1 −3 16 1 3 1 −1 1 3 3 3 −1 −1 3 −1 17 −3 1 1 3 −3 3 −3 −3 3 1 3 −1 18 −3 3 1 1 −3 1 −3 −3 −1 −1 1 −3 19 −1 3 1 3 1 −1 −1 3 −3 −1 −3 −1 20 −1 −3 1 1 1 1 3 1 −1 1 −3 −1 21 −1 3 −1 1 −3 −3 −3 −3 −3 1 −1 −3 22 1 1 −3 −3 −3 −3 −1 3 −3 1 −3 3 23 1 1 −1 −3 −1 −3 1 −1 1 3 −1 1 24 1 1 3 1 3 3 −1 1 −1 −3 −3 1 25 1 −3 3 3 1 3 3 1 −3 −1 −1 3 26 1 3 −3 −3 3 −3 1 −1 −1 3 −1 −3 27 −3 −1 −3 −1 −3 3 1 −1 1 3 −3 −3 28 −1 3 −3 3 −1 3 3 −3 3 3 −1 −1 29 3 −3 −3 −1 −1 −3 −1 3 −3 3 1 −1

Table 2 is an example where φ(n) is defined when M_(sc) ^(RS)=2*N_(sc) ^(RB).

TABLE 2 φ(0), . . . , φ(23) 0 −1 3 1 −3 3 −1 1 3 −3 3 1 3 −3 3 1 1 −1 1 3 −3 3 −3 −1 −3 1 −3 3 −3 −3 −3 1 −3 −3 3 −1 1 1 1 3 1 −1 3 −3 −3 1 3 1 1 −3 2 3 −1 3 3 1 1 −3 3 3 3 3 1 −1 3 −1 1 1 −1 −3 −1 −1 1 3 3 3 −1 −3 1 1 3 −3 1 1 −3 −1 −1 1 3 1 3 1 −1 3 1 1 −3 −1 −3 −1 4 −1 −1 −1 −3 −3 −1 1 1 3 3 −1 3 −1 1 −1 −3 1 −1 −3 −3 1 −3 −1 −1 5 −3 1 1 3 −1 1 3 1 −3 1 −3 1 1 −1 −1 3 −1 −3 3 −3 −3 −3 1 1 6 1 1 −1 −1 3 −3 −3 3 −3 1 −1 −1 1 −1 1 1 −1 −3 −1 1 −1 3 −1 −3 7 −3 3 3 −1 −1 −3 −1 3 1 3 1 3 1 1 −1 3 1 −1 1 3 −3 −1 −1 1 8 −3 1 3 −3 1 −1 −3 3 −3 3 −1 −1 −1 −1 1 −3 −3 −3 1 −3 −3 −3 1 −3 9 1 1 −3 3 3 −1 −3 −1 3 −3 3 3 3 −1 1 1 −3 1 −1 1 1 −3 1 1 10 −1 1 −3 −3 3 −1 3 −1 −1 −3 −3 −3 −1 −3 −3 1 −1 1 3 3 −1 1 −1 3 11 1 3 3 −3 −3 1 3 1 −1 −3 −3 −3 3 3 −3 3 3 −1 −3 3 −1 1 −3 1 12 1 3 3 1 1 1 −1 −1 1 −3 3 −1 1 1 −3 3 3 −1 −3 3 −3 −1 −3 −1 13 3 −1 −1 −1 −1 −3 −1 3 3 1 −1 1 3 3 3 −1 1 1 −3 1 3 −1 −3 3 14 −3 −3 3 1 3 1 −3 3 1 3 1 1 3 3 −1 −1 −3 1 −3 −1 3 1 1 3 15 −1 −1 1 −3 1 3 −3 1 −1 −3 −1 3 1 3 1 −1 −3 −3 −1 −1 −3 −3 −3 −1 16 −1 −3 3 −1 −1 −1 −1 1 1 −3 3 1 3 3 1 −1 1 −3 1 −3 1 1 −3 −1 17 1 3 −1 3 3 −1 −3 1 −1 −3 3 3 3 −1 1 1 3 −1 −3 −1 3 −1 −1 −1 18 1 1 1 1 1 −1 3 −1 −3 1 1 3 −3 1 −3 −1 1 1 −3 −3 3 1 1 −3 19 1 3 3 1 −1 −3 3 −1 3 3 3 −3 1 −1 1 −1 −3 −1 1 3 −1 3 −3 −3 20 −1 −3 3 −3 −3 −3 −1 −1 −3 −1 −3 3 1 3 −3 −1 3 −1 1 −1 3 −3 1 −1 21 −3 −3 1 1 −1 1 −1 1 −1 3 1 −3 −1 1 −1 1 −1 −1 3 3 −3 −1 1 −3 22 −3 −1 −3 3 1 −1 −3 −1 −3 −3 3 −3 3 −3 −1 1 3 1 −3 1 3 3 −1 −3 23 −1 −1 −1 −1 3 3 3 1 3 3 −3 1 3 −1 3 −1 3 3 −3 3 1 −1 3 3 24 1 −1 3 3 −1 −3 3 −3 −1 −1 3 −1 3 −1 −1 1 1 1 1 −1 −1 −3 −1 3 25 1 −1 1 −1 3 −1 3 1 1 −1 −1 −3 1 1 −3 1 3 −3 1 1 −3 −3 −1 −1 26 −3 −1 1 3 1 1 −3 −1 −1 −3 3 −3 3 1 −3 3 −3 1 −1 1 −3 1 1 1 27 −1 −3 3 3 1 1 3 −1 −3 −1 −1 −1 3 1 −3 −3 −1 3 −3 −1 −3 −1 −3 −1 28 −1 −3 −1 −1 1 −3 −1 −1 1 −1 −3 1 1 −3 1 −3 −3 3 1 1 −1 3 −1 −1 29 1 1 −1 −1 −3 −1 3 −1 3 −1 1 3 1 −1 3 1 3 −3 −3 1 −1 −1 1 3

Hopping of a reference signal may be applied as follows.

The sequence group index u of a slot index n_(s) may be defined based on a group hopping pattern f_(gh)(n_(s)) and a sequence shift pattern f_(ss) according to Equation 7.

u=(f _(gh)(n _(s))mod 30   [Equation 7]

17 different group hopping patterns and 30 different sequence shift patterns may exist. Whether to apply group hopping may be indicated by a higher layer.

A PUCCH and a PUSCH may have the same group hopping pattern. A group hopping pattern f_(gh)(n_(s)) may be defined by Equation 8.

$\begin{matrix} {{f_{gh}\left( n_{s} \right)} = \left\{ \begin{matrix} 0 & {{if}\mspace{14mu} {group}\mspace{14mu} {hopping}\mspace{14mu} {is}\mspace{14mu} {disabled}} \\ {\left( {\sum\limits_{i = 0}^{7}{{c\left( {{8n_{s}} + i} \right)} \cdot 2^{i}}} \right){mod}\; 30} & {{if}\mspace{14mu} {group}\mspace{14mu} {hopping}\mspace{14mu} {enabled}} \end{matrix} \right.} & \left\lbrack {{Equation}\mspace{14mu} 8} \right\rbrack \end{matrix}$

In Equation 8, c(i) is a pseudo random sequence that is a PN sequence and may be defined by a Gold sequence of a length-31. Equation 9 shows an example of a gold sequence e(n).

c(n)=(x ₁(n+N _(c))+x ₂(+N _(c)))mod 2

x ₁(n+31)=(x ₁(n+3)+x ₁(n))mod 2

x ₂(n+31)=(x ₂(n+3)+x ₂(n+2)+x ₁(n+1)+x ₁(n))mod 2   [Equation 9]

Here, Nc-1600, x₁(i) is a first m-sequence, and x₂(i) is a second m-sequence. For example, the first m-sequence or the second m-sequence may be initialized according to a cell identifier (ID) for every OFDM symbol, a slot number within one radio frame, an OFDM symbol index within a slot, and the type of a CP. A pseudo random sequence generator may be initialized to

$c_{init} = \left\lfloor \frac{N_{ID}^{cell}}{30} \right\rfloor$

in the first of each radio frame.

A PUCCH and a PUSCH may have the same sequence shift pattern. The sequence shift pattern of the PUCCH may be f_(ss) ^(PUCCH)=N_(ID) ^(cell) mod 30. The sequence shift pattern of the PUSCH may be f_(ss) ^(PUSCH)=(f_(ss) ^(PUCCH)+Δ_(ss))mod 30 and Δ_(ss) ∈ {0,1, . . . , 29} may be configured by a higher layer.

Sequence hopping may be applied to only a reference signal sequence having a length longer than 6N_(sc) ^(RB). Here, a basic sequence index v within a basic sequence group of a slot index n may be defined by Equation 10.

$\begin{matrix} {v = \left\{ \begin{matrix} {c\left( n_{s} \right)} & {{if}\mspace{14mu} {group}\mspace{14mu} {hopping}\mspace{14mu} {is}\mspace{14mu} {disabled}\mspace{14mu} {and}\mspace{14mu} {sequence}\mspace{14mu} {hopping}\mspace{14mu} {is}\mspace{14mu} {enabled}} \\ 0 & {otherwise} \end{matrix} \right.} & \left\lbrack {{Equation}\mspace{14mu} 10} \right\rbrack \end{matrix}$

c(i) may be represented by an example of Equation 9. Whether to apply sequence hopping may be indicated by a higher layer. A pseudo random sequence generator may be initialized to

$c_{init} = {{\left\lfloor \frac{N_{ID}^{cell}}{30} \right\rfloor \cdot 2^{5}} + f_{ss}^{PUSCH}}$

in the first of each radio frame.

A DMRS sequence for a PUSCH may be defined by Equation 11.

r ^(PUSCH)(m·M _(sc) ^(RS) +n)=r _(u,v) ^((α))(n)   [Equation 11]

In Equation 11, m=0,1, . . . and n=0, . . . , M_(sc) ^(RS)−1. M_(sc) ^(RS)=M_(sc) ^(PUSCH).

α=2πn_(cs)/12, that is, a cyclic shift value is given within a slot, and n_(cs) may be defined by Equation 12.

n_(cs)=(n _(DMRS) ⁽¹⁾ +n _(DMRS) ⁽²⁾ +n _(PRS)(n _(s)))mod 12   [Equation 12]

In Equation 12, n_(DMRS) ⁽¹⁾ is indicated by a parameter transmitted by a higher layer, and Table 3 shows an example of a corresponding relationship between the parameter arid n_(DMRS) ⁽¹⁾.

TABLE 3 Parameter n_(DMRS) ⁽¹⁾ 0 0 1 2 2 3 3 4 4 6 5 8 6 9 7 10

Back in Equation 12, n_(DMRS) ⁽²⁾ may be defined by a cyclic shift field within a DCI format 0 for a transmission block corresponding to PUSCH transmission. The DCI format is transmitted in a PDCCH. The cyclic shift field may have a length of 3 bits.

Table 4 shows an example of a corresponding relationship between the cyclic shift field and n_(DMRS) ⁽²⁾.

TABLE 4 Cyclic shift field in DCI format 0 n_(DMRS) ⁽²⁾ 000 0 001 6 010 3 011 4 100 2 101 8 110 10 111 9

If a PDCCH including the DCI format 0 is not transmitted in the same transmission block, if the first PUSCH is semi-persistently scheduled in the same transmission block, or if the first PUSCH is scheduled by a random access response grant in the same transmission block, n_(DMRS) ⁽²⁾ may be 0.

n_(PRS)(n_(s)) may be defined by Equation 13.

n _(PRS)(n _(s))=Σ_(i=0) ⁷ c(8N _(symb) ^(UL) ·n _(s) +i)·2^(i)   [Equation 13]

c(i) may be represented by the example of Equation 9 and may be applied in a cell-specific way of c(i). A pseudo random sequence generator may be initialized to

$c_{init} = {{\left\lfloor \frac{N_{ID}^{cell}}{30} \right\rfloor \cdot 2^{5}} + f_{ss}^{PUSCH}}$

in the first of each radio frame.

A DMRS sequence r^(PUSCH) is multiplied by an amplitude scaling factor β_(PUSCH) and mapped to a physical transmission block, used in relevant PUSCH transmission, from r^(PUSCH)(0) in a sequence starting. The MARS sequence is mapped to a fourth OFDM symbol (OFDM symbol index 3) in case of a normal CP within one slot and mapped to a third OFDM symbol (OFDM symbol index 2) within one slot in case of an extended CP.

An SRS sequence r_(SRS)(n)=r_(u,v) ^((α))(n) is defined. u indicates a PUCCH sequence group index, and v indicates a basic sequence index. The cyclic shift value a is defined by Equation 14.

$\begin{matrix} {\alpha = {2\pi \frac{n_{SRS}^{cs}}{8}}} & \left\lbrack {{Equation}\mspace{14mu} 14} \right\rbrack \end{matrix}$

n_(SRS) ^(cs) is a value configured by a higher layer in related to each UE and may be any one of integers from 0 to 7.

Meanwhile, an orthogonal code cover (OCC) may be applied to a reference signal sequence. The OCC means a code which has different orthogonality and may apply to a sequence. In general, in order to distinguish a plurality of channels from each other, different sequences may be used, but the plurality of channels may be distinguished from each other using the OCC.

The OCC may be used for the following purposes.

1) The OCC may be applied in order to increase the amount of radio resources allocated to an uplink reference signal.

For example, assuming that the cyclic shift values of reference signals transmitted in a first slot and a second slot are allocated as a, a sign (−) may be allocated to the reference signal transmitted in the second slot. That is, a first user may send a reference signal, having a cyclic shift value of a and a sign (+), in the second slot, and a second user may send a reference signal, having a cyclic shift value of a and a sign (−), in the second slot. A BS may estimate the channel of the first user by adding the reference signal transmitted in the second slot and the reference signal transmitted in the first slot. Furthermore, the BS may estimate the channel of the second user by subtracting the reference signal transmitted in the second slot from the reference signal transmitted in the first slot. That is, if the OCC is applied, the BS can distinguish the reference signal transmitted by the first user from the reference signal transmitted by the second user. Accordingly, the amount of radio resources can be doubled because at least two users use different OCCs while using the same reference signal sequence.

2) The OCC may be applied in order to increase an interval between cyclic shift values allocated to the multiple antennas or the multiple layers of a single user. Cyclic shift values allocated to multiple layers are described below, but cyclic shift values allocated to multiple antennas may also be applied.

A uplink reference signal distinguishes channels from each other based on cyclic shift values. In order to distinguish a plurality of layers from each other in a multi-antenna system, different cyclic shift values may be allocated to reference signals for respective layers. The number of cyclic shift values to be allocated must be increased according to an increase of the number of layers, and thus an interval between the cyclic shift values is reduced. Accordingly, channel estimation performance is reduced because it is difficult to distinguish a plurality of channels from each other. In order to overcome this problem, the OCC may be applied to each layer. For example, it is assumed that cyclic shift offsets 0, 6, 3, and 9 are allocated to the respective reference signals of four layers. An interval between the cyclic shift values of the reference signals of the respective layers is 3. Here, the interval between the cyclic shift values of the reference signals of the layers of antennas may be increased to 6 by applying an OCC of a sign (−) to a third layer and a fourth layer. Accordingly, the performance of channel estimation can be increased.

3) The OCC may be applied in order to increase an interval between cyclic shift values allocated to a single user.

In an MU-MIMO system including a plurality of users having multiple antennas, the OCC may be applied to a cyclic shift value. For example, from a viewpoint of a single user performing MIMO transmission, in order to distinguish a plurality of antennas or a plurality of layers from each other, a cyclic shift value having a distance interval between antennas or layers may be applied. From a viewpoint of multiple users, however, the cyclic shift interval between the users may be narrowed. In order to overcome this problem, the OCC may be used. When the OCC is applied, the same cyclic shift value may be applied between the multiple users according to a type of the OCC.

FIG. 13 shows an example where the OCC is applied to a reference signal.

Both a reference signal sequence for a layer 0 and a reference signal sequence for a layer 1 within one subframe are mapped to the fourth SC-FDMA symbol of a first slot and the fourth SC-FDMA symbol of a second slot. The same sequence is mapped to two SC-FDMA symbols in each layer. Here, the reference signal sequence for the layer 0 is multiplied by an orthogonal sequence [+1 +1] and then mapped to the SC-FDMA symbol. The reference signal sequence for the layer 1 is multiplied by an orthogonal sequence [++1 −1] and then mapped to the SC-FDMA symbol. That is, when the reference signal sequence for the layer 1 is mapped to the second slot within the one subframe, the reference signal sequence is multiplied by −1 and then mapped.

If the OCC is applied as described above, a BS that receives a reference signal may estimate the channel of the layer 0 by adding a reference signal sequence transmitted in the first slot and a reference signal sequence transmitted in the second slot. Furthermore, the BS may estimate the channel of the layer 1 by subtracting the reference signal sequence transmitted in the second slot from the reference signal sequence transmitted in the first slot. That is, a BS can distinguish reference signals, transmitted in respective layers, from each other by applying the OCC. Accordingly, a plurality of reference signals can be transmitted using the same resources. If the number of possible cyclic shift values is 6, the number of layers or users that may be multiplexed using the OCC can be increased up to 12.

In this example, it is assumed that the binary format [+1 +1] or [+1 or −1] is used as the OCC, but not limited thereto and various kinds of orthogonal sequences may be used as the OCC. For example, orthogonal sequences, such as Walsh codes, DFT coefficients, and CAZAC sequences, may be applied to the OCC. Furthermore, reference signals can be multiplexed more easily between users having different bandwidths by applying the OCC.

A proposed method of generating a reference signal sequence is described below.

As described above, whether to perform sequence group hopping (SGH) on a reference signal sequence in LTE rel-8 may be indicated by a signal that is transmitted in a cell-specific way. The cell-specific signal indicating whether to perform SGH on a reference signal sequence is hereinafter called a cell-specific GH parameter. Although LTE rel-8 UE and LTE-A UE coexist within a cell, whether to perform SGH on a reference signal sequence is the same in the LTE Rel-8 UE and the LTE-A UE. Currently defined SGH or sequence gopping (SH) may be performed for every slot. The cell-specific GH parameter may be a group-hopping-enabled parameter provided by a higher layer. When the value of the group-hopping-enabled parameter is true, SGH for a reference signal sequence is performed, but SH is not performed. When the value of the group-hopping-enabled parameter is false, SGH for a reference signal sequence is not performed, and whether to perform SH is determined by a cell-specific SH parameter, provided by a higher layer and indicating whether to perform SH. The cell-specific SH parameter may be a sequence-hopping-enabled parameter provided by a higher layer.

Meanwhile, in LTE-A, LTE rel-8 UE and LTE-A UE may perform MU-MIMO transmission, or LTE-A UEs may perform MU-MIMO transmission. Here, in order to support the MU-MIMO transmission of UEs having different bandwidths, the OCC may be applied. When the OCC is applied, orthogonality between the UEs performing the MU-MIMO transmission can be improved and the throughput can also be improved. However, if UEs have different bandwidths and whether to perform SGH or SH for a reference signal sequence is determined by a cell-specific GH or SH parameter defined in LTE rel-8, orthogonality between reference signals transmitted by the respective UEs may not be sufficiently guaranteed.

FIG. 14 is an example where a plurality of UEs performs MU-MIMO transmission using different bandwidths. In FIG. 14( a), a first UE UE1 and a second UE UE2 perform the same bandwidth. In this case, whether to perform SGH or SH for the base sequence of a reference signal may be determined by a cell-specific GH or SH parameter defined in LTE rel-8. In FIG. 14( b), a first UE UE1 uses a bandwidth which is the sum of bandwidths used by a second UE UE2 and a third UE UE3. That is, the first UE, the second UE, and the third UE use different bandwidths. In this case, whether to perform SGH or SH for the base sequence of a reference signal transmitted by each UE needs to be determined using a new method.

Accordingly, a UE-specific SGH parameter may be newly defined in addition to the existing cell-specific GH parameter and the existing cell-specific SH parameter. The UE-specific SGH parameter is information for specific UE and may be transmitted to only the specific UE. The UE-specific SGH parameter may be applied to a DMRS transmitted using PUSCH resources allocated to specific UE. That is, the UE-specific SGH parameter may indicate whether to perform SGH/SH for the base sequence of a DMRS that is transmitted using PUSCH resources. For convenience of description hereinafter, only an example where whether to perform SGH and SH for the base sequence of a reference signal is determined by the UE-specific SGH parameter is described, but not limited thereto. Whether to apply SH for the base sequence of the reference signal may be determined by a UE-specific SH parameter different from the UE-specific SGH parameter. Furthermore, an example where the present invention is applied to the base sequence of a DMRS transmitted using PUSCH resources is described, but not limited thereto. The present invention may also be applied to a DMRS, an SRS, etc. which are transmitted using PUCCH resources in various ways. Furthermore, an MU-MIMO environment in which a plurality of UEs has different bandwidths is assumed, but the present invention may be applied to an MU-MIMO or SU-MIMO environment in which a plurality of UEs has the same bandwidth.

When a value of the cell-specific GH parameter or the cell-specific SH parameter is true and thus SGH or SH is performed on the base sequence of a reference signal, SGH or SH of a slot level is in common performed on a DMRS using PUSCH resources and a DMRS and STS using PUCCH resources. That is, the sequence group index (or number) of the base sequence of the reference signal is changed for every slot, or a base sequence index (or number) is changed within a sequence group. Here, whether SGH or SH will be performed on the DMRS using PUSCH resources may be indicated by a UE-specific SGH parameter again. In other words, the UE-specific SGH parameter overrides the cell-specific GH parameter or the cell-specific SGH parameter. The UE-specific SGH parameter may be a disable sequence-group hopping parameter. That is, if a value of the UE-specific SGH parameter is true, SGH and SH may not be performed irrespective of the cell-specific GH parameter or the cell-specific SH parameter. More particularly, when a value of the UE-specific SCH parameter is true, SGH and SH for the base sequence of a reference signal may not be performed although to execute SGH or SH for the base sequence of the reference signal is indicated by the cell-specific GH parameter or the cell-specific SR parameter. If SGH is not performed, a sequence group index of the base sequence of the reference signal may not be changed for every slot. Furthermore, as in the case where SGH is performed by the cell-specific GH parameter, a base sequence index of the base sequence of the reference signal is not changed for every slot because SH is not performed. Here, two slots within a subframe send the base sequences of the reference signals of base sequence indices, such as the same sequence group index, because SGH and SH are not performed only within one subframe, but SGH or SH may be applied between subframes. Alternatively, since SGH and SH are not applied within all subframes, all slots may send the base sequences of the reference signals of the same sequence group index and the same base sequence index. Meanwhile, when a value of the UE-specific SGH parameter is false, SGH or SH for the base sequence of a reference signal may be performed according to the cell-specific GH parameter or the cell-specific SH parameter.

FIG. 15 is an example where SGH and SH are not performed by the proposed UE-specific SGH parameter. Referring to FIG. 15, when SGH and SH are performed in LTE rel-8 or 9, a sequence group index or a base sequence index of base sequence of a reference signal transmitted in each slot is different. Approach 1 is a case where SGH and SH are not performed within a subframe according to a UE-specific SGH parameter. Two slots within each subframe generate the base sequences of the reference signals having the same sequence group index and the same base sequence index, and a sequence group index or a base sequence index is changed between the subframes. Approach 2 is a case where SGH and SH are not performed within all subframes according to a UE-specific SGH parameter. Accordingly, all the subframes generate the base sequences of the reference signals having the same sequence group index and the same base sequence index.

FIG. 16 is an embodiment of a proposed method of generating a reference signal sequence.

At step S100, UE receives a UE-specific SGH parameter. The UE-specific SGH parameter may be given by a higher layer. At step S110, the UE generates a reference signal sequence based on a base sequence for every slot. The base sequence may be classified according to a sequence-group number and a base sequence number which are determined for every slot by the UE-specific SGH parameter indicating whether to perform SGH and SH.

The UE may be informed of whether to perform SGH and SH according to the UE-specific SGH parameter using various methods described below.

1) A frequency hopping flag included in a DCI format for uplink transmission may play the role of the UE-specific SGH parameter. For example, if frequency hopping is enabled by the frequency hopping flag, SGH or SH of a slot level may be performed. Furthermore, if the frequency hopping is disabled by the frequency hopping flag, SGH and SH for the base sequence of a DMRS using PUSCH resources may not be performed. Alternatively, SGH or SH may be performed for every subframe.

2) Whether to perform SGH and SH may be indicated by masking information, indicating whether to perform SGH and SH for the base sequence of a reference signal, in a bit indicating a UE ID included in a DCI format for uplink transmission.

3) Whether to perform SGH and SH for the base sequence of a reference signal may be indicated when a specific index of a cyclic shift indicator included in a DCI format for UL transmission is designated.

4) A UE-specific SGH parameter indicating whether to perform SGH and SH for the base sequence of a reference signal may be included in a DCI format for UL transmission.

5) A UE-specific SGH parameter may be transmitted to specific UE through higher layer signaling for the specific UE.

6) If a clustered DFT-s OFDM transmission scheme is used, SGH and SH for the base sequence of a reference signal may not be performed.

Meanwhile, when SGH and SH for the base sequence of a reference signal are not performed according to a UE-specific SGH parameter, an OCC may be applied to the relevant reference signal. If SGH or SH for the base sequence of the reference signal is performed, the OCC may not be applied.

A variety of methods may be used in order to indicate whether to apply the OCC. First, when a cyclic shift index is indicated through a DCI format and an OCC index indicating whether to apply an OCC is transmitted through a higher layer, if SGH and SH for the base sequence of a reference signal are not performed, whether to apply an OCC according to an OCC index may be used without change. For example, an OCC may not be applied if an OCC index is 0, and an OCC may be applied if an OCC index is 1. Alternatively, an OCC may not be applied if an OCC index is 1, and an OCC may be applied if an OCC index is 0. Furthermore, if SGH or SH for the base sequence of a reference signal is preformed, whether to apply an OCC may be determined in an opposite way to the OCC index. For example, an OCC may be applied if an OCC index is 0, and an OCC may not be applied if an OCC index is 1. Alternatively, an OCC may be applied if an DCC index is 1, and an OCC may not be applied if an OCC index is 0.

Alternatively, an OCC index indicating whether to apply an OCC may not be separately defined, but a specific OCC may be indicated so that the specific OCC is applied to specific cyclic shift index by combining a cyclic shift index and an OCC index of 3 bits within a DCI format. Here, if SGH for the base sequence of a reference signal is performed, an OCC index indicated by a relevant cyclic shift index may be reversed again, so that the OCC is not applied. Furthermore, if SGH and SH for the base sequence of a reference signal is not performed according to a UE-specific SGH parameter, the OCC may be applied by using an OCC index indicated by a relevant cyclic shift index without change. Accordingly, interference between reference signals allocated to respective layers can be reduced.

In the above description, an example where whether to perform SGH and SH for the base sequence of a reference signal is determined by a UE-specific SGH parameter has been described. In an MU-MIMO environment, however, in order to further guarantee orthogonality between the reference signals of UEs, a new parameter indicating whether to perform SH may be further defined. The new parameter indicating whether to perform SH may be a UE-specific SH parameter. The UE-specific SH parameter may be applied using the same method as the UE-specific SGH parameter. That is, the UE-specific SH parameter may override a cell-specific SH parameter. The above-described UE-specific SGH parameter may determine only whether to perform SGH. That is, when a value of the UE-specific SGH parameter is true, SGH for the base sequence of a reference signal is not performed. Furthermore, whether to perform SH for the base sequence of the reference signal is determined by the UE-specific SH parameter. When a value of the UE-specific SH parameter is true, SH for the base sequence of the reference signal is not performed. When a value of the UE-specific SH parameter is false, whether to perform SH for the base sequence of the reference signal may be determined by a cell-specific SH parameter. The UE-specific SH parameter may be dynamically signalized implicitly or explicitly using signaling through a PDCCH or may be given by a higher layer, such as RRC signaling, implicitly or explicitly.

Meanwhile, in the above description, it has been described that the UE-specific SGH parameter, the UE-specific GH parameter, or the UE-specific SH parameter override the cell-specific GH parameter or the cell-specific SH parameter, irrespective of a UL transmission mode, but may be changed according to a transmission mode. In LTE rel-8/9, a single antenna transmission mode is basically supported. In LTE-A, however, a multi-antenna transmission mode, a transmission mode for discontinuous allocation, etc. may be defined for the efficiency of UL transmission. Here, whether to perform the UE-specific SGH parameter, the UE-specific GH parameter, or the UE-specific SH parameter may be determined according to the transmission mode. For example, in the single antenna transmission mode, although the UE-specific SGH parameter overrides the cell-specific GH parameter or the cell-specific SH parameter, whether to perform SGH or SH for the base sequence of a reference signal may be determined by the cell-specific GH parameter or the cell-specific SH parameter.

FIG. 17 is a block diagram showing a BS and UE in which the embodiments of the present invention are implemented.

The BS 800 includes a processor 810, memory 820, and a radio frequency (RF) unit 830. The processor 810 implements the proposed functions, processes, and/or methods. The layers of a wireless interface protocol may be implemented by the processor 810. The memory 820 is coupled to the processor 810, and it stores various pieces of information for driving the processor 810. The RF unit 830 is coupled to the processor 810, and it sends a UE-specific SGH parameter to UE.

The UE 900 includes a processor 910, memory 920, and an RF unit 930. The RF unit 930 is coupled to the processor 910, and it receives a UE-specific SGH parameter. The processor 910 implements the proposed functions, processes, and/or methods. The layers of a wireless interface protocol may be implemented by the processor 910. The processor 910 is configured to generate a reference signal sequence based on a base sequence for every slot. The base sequence is classified according to a sequence-group number and a base sequence number which are determined for every slot by a UE-specific SGH parameter indicating whether to perform SGH. The memory 920 is coupled to the processor 910, and it stores various pieces of information for driving the processor 910.

The processors 810, 910 may include application-specific integrated circuit (ASIC), other chipset, logic circuit and/or data processing device. The memories 820, 920 may include read-only memory (ROM), random access memory (RAM), flash memory, memory card, storage medium and/or other storage device. The RF units 830, 930 may include baseband circuitry to process radio frequency signals. When the embodiments are implemented in software, the techniques described herein can be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein. The modules can be stored in memories 820, 920 and executed by processors 810, 910. The memories 820, 920 can be implemented within the processors 810, 910 or external to the processors 810, 910 in which case those can be communicatively coupled to the processors 810, 910 via various means as is known in the art.

In view of the exemplary systems described herein, methodologies that may be implemented in accordance with the disclosed subject matter have been described with reference to several flow diagrams. While for purposed of simplicity, the methodologies are shown and described as a series of steps or blocks, it is to be understood and appreciated that the claimed subject matter is not limited by the order of the steps or blocks, as some steps may occur in different orders or concurrently with other steps from what is depicted and described herein. Moreover, one skilled in the art would understand that the steps illustrated in the flow diagram are not exclusive and other steps may be included or one or more of the steps in the example flow diagram may be deleted without affecting the scope and spirit of the present disclosure.

What has been described above includes examples of the various aspects. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the various aspects, but one of ordinary skill in the art may recognize that many further combinations and permutations are possible. Accordingly, the subject specification is intended to embrace all such alternations, modifications and variations that fall within the spirit and scope of the appended claims. 

What is claimed is:
 1. A method of receiving, by a base station, a reference signal in a wireless communication system, the method comprising: transmitting a cell-specific sequence group hopping parameter to a plurality of user equipments (UEs) in a cell, wherein the cell-specific sequence group hopping parameter is used to enable a sequence group hopping for the plurality of UEs in the cell; transmitting a UE-specific sequence group hopping parameter to a certain UE, among the plurality of UEs, wherein the UE-specific sequence group hopping parameter is used to disable the sequence group hopping, enabled by the cell-specific SGH parameter, for the certain UE; and receiving a reference signal, which is generated based on a sequence group number, from the certain UE, wherein the sequence group number is determined by the UE-specific sequence group hopping parameter.
 2. The method of claim 1, wherein the cell-specific sequence group hopping parameter is a Group-hopping-enabled parameter transmitted through a high layer.
 3. The method of claim 1, wherein the EX-specific sequence group hopping parameter is a Disable-sequence-group-hopping parameter transmitted through a high layer.
 4. The method of claim 1, wherein the sequence group number is identical with each of other sequence group numbers in a slot.
 5. The method of claim 1, wherein the sequence group number is defined by a group hopping pattern and a sequence shift pattern in each of a plurality of slots.
 6. The method of claim 5, wherein the group hopping pattern is set to 0 according to the UE-specific sequence group hopping parameter.
 7. The method of claim 1, wherein the sequence group number is determined by an Equation below: u=(f _(gh)(n _(s)) mod30 where n_(s) denotes a slot number in a frame, f_(ss) denotes a sequence shift pattern configured by a cell identifier (ID) and a higher layer, and f_(gh)(n_(s)) is a group hopping pattern which is set to 0 according to the UE-specific sequence group hopping parameter.
 8. A base station in a wireless communication system, the base station comprising: a radio frequency (RF) unit configured to transmit or receive a radio signal; and a processor coupled to the RF unit, and configured to: transmit a cell-specific sequence group hopping parameter to a plurality of user equipments (UEs) in a cell, wherein the cell-specific sequence group hopping parameter is used to enable a sequence group hopping for the plurality of UEs in the cell, transmit a UE-specific sequence group hopping parameter to a certain UE, among the plurality of UEs, wherein the UE-specific sequence group hopping parameter is used to disable the sequence group hopping, enabled by the cell-specific SGH parameter, for the certain UE, and receive a reference signal, which is generated based on a sequence group number, from the certain UE, wherein the sequence group number is determined by the UE-specific sequence group hopping parameter.
 9. The base station of claim 8, wherein the cell-specific sequence group hopping parameter is a Group-hopping-enabled parameter transmitted through a high layer.
 10. The base station of claim 8, wherein the UE-specific sequence group hopping parameter is a Disable-sequence-group-hopping parameter transmitted through a high layer.
 11. The base station of claim 8, wherein the sequence group number is identical with each of other sequence group numbers in a slot.
 12. The base station of claim 8, wherein the sequence group number is defined by a group hopping pattern and a sequence shift pattern in each of a plurality of slots.
 13. The base station of claim 12, wherein the group hopping pattern is set to 0 according to the UE-specific sequence group hopping parameter.
 14. The base station of claim 8, wherein the sequence group number is determined by an Equation below: u=(f _(gh)(n _(s))+f _(ss))mod 30, where n_(s) denotes a slot number in a frame, denotes a sequence shift pattern configured by a cell identifier (ID) and a higher layer, and f_(gh)(n_(s)) is a group hopping pattern which is set to 0 according to the UE-specific sequence group hopping parameter. 